Multi-resistance mram

ABSTRACT

Apparatuses, systems, and methods are disclosed for magnetoresistive random access memory. A magnetic tunnel junction for storing data may include a reference layer, a barrier layer, and a free layer. A barrier layer may be disposed between a reference layer and a free layer. A free layer may include a nucleation region and an arm. A nucleation region may be configured to form a magnetic domain wall. An arm may be narrower than a nucleation region and may extend from the nucleation region. An arm may include a plurality of pinning sites formed at predetermined locations along the arm for pinning a domain wall.

TECHNICAL FIELD

The present disclosure, in various embodiments, relates tomagnetoresistive random access memory and more particularly relates tomulti-resistance magnetoresistive random access memory.

BACKGROUND

Various types of magnetoresistive random access memory (MRAM) store datausing magnetic tunnel junctions. A magnetic tunnel junction (MTJ) mayinclude “fixed” and “free” magnetic layers, where a magnetic moment ofthe free layer may be switched to be parallel or antiparallel to amagnetic moment of the fixed layer. A thin dielectric or barrier layermay separate the fixed and free layers, and current may flow across thebarrier layer due to quantum tunneling. A difference in resistancebetween parallel and antiparallel states allows data to be stored. Forexample, a low resistance may correspond to a binary “1” and a highresistance may correspond to a binary “0,” Alternatively, a lowresistance may correspond to a binary “0” and a high resistance maycorrespond to a binary “1.” However, a memory device that uses an arrayof MTJs to store one bit of data per MTJ may have a low storage density,or a low capacity for the area the array occupies.

SUMMARY

Apparatuses are presented for magnetoresistive random access memory. Inone embodiment, a magnetic tunnel junction for storing data includes areference layer, a barrier layer, and a free layer. In a certainembodiment, a barrier layer is disposed between a reference layer and afree layer. In a further embodiment, a free layer includes a nucleationregion configured to form a magnetic domain wall. In some embodiments, afree layer includes an arm. In certain embodiments, an arm may benarrower than a nucleation region and may extend from the nucleationregion. In further embodiments, an arm may include a plurality ofpinning sites formed at predetermined locations along the arm forpinning a domain wall.

Systems are presented for neuromorphic computing. In one embodiment, asystem includes a neuromorphic computing die. In a certain embodiment, aneuromorphic computing die includes a plurality of artificial neuronsand a synapse array of multi-state magnetic memory cells coupling theartificial neurons. In a further embodiment, a magnetic storage layerfor a multi-state magnetic memory cell includes a pad-shaped region forforming a magnetic domain wall. In one embodiment, a magnetic storagelayer for a multi-state magnetic memory cell includes one or more arms.In a certain embodiment, one or more arms may be narrower than apad-shaped region, and may extend from the pad-shaped region. In afurther embodiment, one or more arms may include a plurality of notchesfor pinning a domain wall.

An apparatus, in another embodiment, includes means for forming amagnetic domain wall in a free layer for a magnetic tunnel junction. Ina certain embodiment, an apparatus includes means for pinning a domainwall at one or more predetermined locations along one or more arms in afree layer.

In another embodiment, a magnetoresistive random access memory dieincludes a plurality of memory cells. In one embodiment, a memory cellincludes a fixed layer, a barrier layer, and a free layer. In a furtherembodiment, a barrier layer is disposed between a fixed layer and a freelayer. A free layer, in one embodiment, includes a nucleation regionconfigured to form a magnetic domain wall in response to a writecurrent. In a certain embodiment, a free layer includes a wall extensionregion configured to provide a plurality of resistance states for amemory cell, corresponding to positions of a domain wall within the wallextension region. In a further embodiment, a free layer includes aplurality of pinning sites formed at predetermined locations in a wallextension region. In certain embodiments, pinning sites may beconfigured to pin a domain wall such that the domain wall migrates fromone pinning site to a subsequent pinning site in response to anincreased write current.

Methods are presented for magnetoresistive random access memory. Amethod, in one embodiment, includes applying a first write current to amagnetic tunnel junction, to form a magnetic domain wall in a nucleationregion of a free layer of the magnetic tunnel junction. In a certainembodiment, a domain wall may migrate to a first pinning site in a freelayer. In a further embodiment, a method may include applying a secondwrite current to a magnetic tunnel junction to move a magnetic domainwall from a first pinning site to a second pinning site in a free layer.In a further embodiment, pinning sites may be formed at predeterminedlocations in a free layer.

BRIEF DESCRIPTION OF THE DRAWINGS

A more particular description is included below with reference tospecific embodiments illustrated in the appended drawings. Understandingthat these drawings depict only certain embodiments of the disclosureand are not therefore to be considered to be limiting of its scope, thedisclosure is described and explained with additional specificity anddetail through the use of the accompanying drawings, in which:

FIG. 1 is a schematic block diagram of one embodiment of a systemcomprising magnetoresistive random access memory (MRAM);

FIG. 2 is a schematic block diagram illustrating one embodiment of anMRAM die;

FIG. 3 is a schematic block diagram illustrating a portion of anartificial neural network, in one embodiment;

FIG. 4 is a is a schematic block diagram illustrating one embodiment ofa neuromorphic computing die;

FIG. 5 is schematic block diagram illustrating one embodiment of amagnetic tunnel junction;

FIG. 6A is a top view illustrating one embodiment of a free layer for amagnetic tunnel junction, in a first resistance state;

FIG. 6B is a top view illustrating the free layer of FIG. 6A, in asecond resistance state;

FIG. 6C is a top view illustrating the free layer of FIG. 6A, in a thirdresistance state;

FIG. 6D is a top view illustrating the free layer of FIG. 6A, in afourth resistance state;

FIG. 7A is a top view illustrating another embodiment of a free layerfor a magnetic tunnel junction, in a first resistance state;

FIG. 7B is a top view illustrating the free layer of FIG. 7A, in asecond resistance state;

FIG. 7C is a top view illustrating the free layer of FIG. 7A, in a thirdresistance state;

FIG. 7D is a top view illustrating the free layer of FIG. 7A, in afourth resistance state;

FIG. 7E is a top view illustrating the free layer of FIG. 7A, in a fifthresistance state;

FIG. 7F is a top view illustrating the free layer of FIG. 7A, in a sixthresistance state;

FIG. 7G is a top view illustrating the free layer of FIG. 7A, in aseventh resistance state;

FIG. 7H is a top view illustrating the free layer of FIG. 7A, in aneighth resistance state;

FIG. 8 is a top view illustrating another embodiment of a free layer fora magnetic tunnel junction;

FIG. 9 is a schematic flow chart diagram illustrating one embodiment ofa method for writing data to magnetoresistive memory; and

FIG. 10 is a schematic flow chart diagram illustrating anotherembodiment of a method for writing data to magnetoresistive memory.

DETAILED DESCRIPTION

Aspects of the present disclosure may be embodied as an apparatus,system, method, or computer program product. Accordingly, aspects of thepresent disclosure may take the form of an entirely hardware embodiment,an entirely software embodiment (including firmware, resident software,micro-code, or the like) or an embodiment combining software andhardware aspects that may all generally be referred to herein as a“circuit,” “module,” “apparatus,” or “system.” Furthermore, aspects ofthe present disclosure may take the form of a computer program productembodied in one or more non-transitory computer readable storage mediastoring computer readable and/or executable program code.

Many of the functional units described in this specification have beenlabeled as modules, in order to more particularly emphasize theirimplementation independence. For example, a module may be implemented asa hardware circuit comprising custom VLSI circuits or gate arrays,off-the-shelf semiconductors such as logic chips, transistors, or otherdiscrete components. A module may also be implemented in programmablehardware devices such as field programmable gate arrays, programmablearray logic, programmable logic devices, or the like.

Modules may also be implemented at least partially in software forexecution by various types of processors. An identified module ofexecutable code may, for instance, comprise one or more physical orlogical blocks of computer instructions which may, for instance, beorganized as an object, procedure, or function. Nevertheless, theexecutables of an identified module need not be physically locatedtogether, but may comprise disparate instructions stored in differentlocations which, when joined logically together, comprise the module andachieve the stated purpose for the module.

Indeed, a module of executable code may include a single instruction, ormany instructions, and may even be distributed over several differentcode segments, among different programs, across several memory devices,or the like. Where a module or portions of a module are implemented insoftware, the software portions may be stored on one or more computerreadable and/or executable storage media. Any combination of one or morecomputer readable storage media may be utilized. A computer readablestorage medium may include, for example, but not limited to, anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, or device, or any suitable combinationof the foregoing, but would not include propagating signals. In thecontext of this document, a computer readable and/or executable storagemedium may be any tangible and/or non-transitory medium that may containor store a program for use by or in connection with an instructionexecution system, apparatus, processor, or device.

Computer program code for carrying out operations for aspects of thepresent disclosure may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Python, Java, Smalltalk, C++, C#, Objective C, or the like,conventional procedural programming languages, such as the “C”programming language, scripting programming languages, and/or othersimilar programming languages. The program code may execute partly orentirely on one or more of a user's computer and/or on a remote computeror server over a data network or the like.

A component, as used herein, comprises a tangible, physical,non-transitory device. For example, a component may be implemented as ahardware logic circuit comprising custom VLSI circuits, gate arrays, orother integrated circuits; off-the-shelf semiconductors such as logicchips, transistors, or other discrete devices; and/or other mechanicalor electrical devices. A component may also be implemented inprogrammable hardware devices such as field programmable gate arrays,programmable array logic, programmable logic devices, or the like. Acomponent may comprise one or more silicon integrated circuit devices(e.g., chips, die, die planes, packages) or other discrete electricaldevices, in electrical communication with one or more other componentsthrough electrical lines of a printed circuit board (PCB) or the like.Each of the modules described herein, in certain embodiments, mayalternatively be embodied by or implemented as a component.

A circuit, as used herein, comprises a set of one or more electricaland/or electronic components providing one or more pathways forelectrical current. In certain embodiments, a circuit may include areturn pathway for electrical current, so that the circuit is a closedloop. In another embodiment, however, a set of components that does notinclude a return pathway for electrical current may be referred to as acircuit (e.g., an open loop). For example, an integrated circuit may bereferred to as a circuit regardless of whether the integrated circuit iscoupled to ground (as a return pathway for electrical current) or not.In various embodiments, a circuit may include a portion of an integratedcircuit, an integrated circuit, a set of integrated circuits, a set ofnon-integrated electrical and/or electrical components with or withoutintegrated circuit devices, or the like. In one embodiment, a circuitmay include custom VLSI circuits, gate arrays, logic circuits, or otherintegrated circuits; off-the-shelf semiconductors such as logic chips,transistors, or other discrete devices; and/or other mechanical orelectrical devices. A circuit may also be implemented as a synthesizedcircuit in a programmable hardware device such as field programmablegate array, programmable array logic, programmable logic device, or thelike (e.g., as firmware, a netlist, or the like). A circuit may compriseone or more silicon integrated circuit devices (e.g., chips, die, dieplanes, packages) or other discrete electrical devices, in electricalcommunication with one or more other components through electrical linesof a printed circuit board (PCB) or the like. Each of the modulesdescribed herein, in certain embodiments, may be embodied by orimplemented as a circuit.

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present disclosure. Thus,appearances of the phrases “in one embodiment,” “in an embodiment,” andsimilar language throughout this specification may, but do notnecessarily, all refer to the same embodiment, but mean “one or more butnot all embodiments” unless expressly specified otherwise. The terms“including,” “comprising,” “having,” and variations thereof mean“including but not limited to” unless expressly specified otherwise. Anenumerated listing of items does not imply that any or all of the itemsare mutually exclusive and/or mutually inclusive, unless expresslyspecified otherwise. The terms “a,” “an,” and “the” also refer to “oneor more” unless expressly specified otherwise.

Aspects of the present disclosure are described below with reference toschematic flowchart diagrams and/or schematic block diagrams of methods,apparatuses, systems, and computer program products according toembodiments of the disclosure. It will be understood that each block ofthe schematic flowchart diagrams and/or schematic block diagrams, andcombinations of blocks in the schematic flowchart diagrams and/orschematic block diagrams, can be implemented by computer programinstructions. These computer program instructions may be provided to aprocessor of a computer or other programmable data processing apparatusto produce a machine, such that the instructions, which execute via theprocessor or other programmable data processing apparatus, create meansfor implementing the functions and/or acts specified in the schematicflowchart diagrams and/or schematic block diagrams block or blocks.

It should also be noted that, in some alternative implementations, thefunctions noted in the block may occur out of the order noted in thefigures. For example, two blocks shown in succession may, in fact, beexecuted substantially concurrently, or the blocks may sometimes beexecuted in the reverse order, depending upon the functionalityinvolved. Other steps and methods may be conceived that are equivalentin function, logic, or effect to one or more blocks, or portionsthereof, of the illustrated figures. Although various arrow types andline types may be employed in the flowchart and/or block diagrams, theyare understood not to limit the scope of the corresponding embodiments.For instance, an arrow may indicate a waiting or monitoring period ofunspecified duration between enumerated steps of the depictedembodiment.

In the following detailed description, reference is made to theaccompanying drawings, which form a part thereof. The foregoing summaryis illustrative only and is not intended to be in any way limiting. Inaddition to the illustrative aspects, embodiments, and featuresdescribed above, further aspects, embodiments, and features will becomeapparent by reference to the drawings and the following detaileddescription. The description of elements in each figure may refer toelements of proceeding figures. Like numbers may refer to like elementsin the figures, including alternate embodiments of like elements.

FIG. 1 depicts a system 100 comprising magnetoresistive random accessmemory (MRAM) 150. In the depicted embodiment, the system includes acomputing device 110. In various embodiments, a computing device 110 maybe any electronic device capable computing by performing arithmetic orlogical operations on electronic data. For example, a computing device110 may be a server, a workstation, a desktop computer, a laptopcomputer, a tablet, a smartphone, a control system for anotherelectronic device, a network attached storage device, a block device ona storage area network, a router, a network switch, or the like. Incertain embodiments, a computing device 110 may include anon-transitory, computer readable storage medium that stores computerreadable instructions configured to cause the computing device 110 toperform steps of one or more of the methods disclosed herein.

In the depicted embodiment, the computing device 110 includes aprocessor 115, a memory 130, and storage 140. In various embodiments, aprocessor 115 may be any electronic element that carries out thearithmetic or logical operations performed by the computing device. Forexample, in one embodiment, the processor 115 may be a general-purposeprocessor that executes stored program code. In another embodiment, aprocessor 115 may be a field-programmable gate array (FPGA), anapplication-specific integrated circuit (ASIC), or the like, thatoperates on data stored by the memory 130 and/or the storage 140. In acertain embodiment, a processor 115 may be a controller for a storagedevice (e.g., on a storage area network) a networking device, or thelike.

In the depicted embodiment, the processor 115 includes a cache 120. Invarious embodiments, a cache 120 may store data for use by the processor115. In certain embodiments, a cache 120 may be smaller and faster thanthe memory 130, and may duplicate data in frequently-used locations ofthe memory 130, or the like. In certain embodiments, a processor 115 mayinclude a plurality of caches 120. In various embodiments, a cache 120may include one or more types of memory media for storing data, such asstatic random access memory (SRAM) 122, magnetoresistive random accessmemory (MRAM) 150, or the like. For example, in one embodiment, a cache120 may include SRAM 122. In another embodiment, a cache 120 may includeMRAM 150. In a certain embodiment, a cache 120 may include a combinationof SRAM 122, MRAM 150, and/or other memory media types.

The memory 130, in one embodiment, is coupled to the processor 115 by amemory bus 135. In certain embodiments, the memory 130 may store datathat is directly addressable by the processor 115. In variousembodiments, a memory 130 may include one or more types of memory mediafor storing data, such as dynamic random access memory (DRAM) 132, MRAM150, or the like. For example, in one embodiment, a memory 130 mayinclude DRAM 132. In another embodiment, a memory 130 may include MRAM150. In a certain embodiment, a memory 130 may include a combination ofDRAM 132, MRAM 150, and/or other memory media types.

The storage 140, in one embodiment, is coupled to the processor 115 by astorage bus 145. In certain embodiments, the storage bus 145 may be aperipheral bus of the computing device 110, such as a peripheralcomponent interconnect express (PCI Express or PCIe) bus, a serialAdvanced Technology Attachment (SATA) bus, a parallel AdvancedTechnology Attachment (PATA) bus, a small computer system interface(SCSI) bus, a FireWire bus, a Fibre Channel connection, a UniversalSerial Bus (USB), a PCIe Advanced Switching (PCIe-AS) bus, or the like.In various embodiments, the storage 140 may store data that is notdirectly addressable by the processor 115, but that may be accessed viaone or more storage controllers. In certain embodiments, the storage 140may be larger than the memory 130. In various embodiments, a storage 140may include one or more types of storage media for storing data, such asa hard disk drive, NAND flash memory 142, MRAM 150, or the like. Forexample, in one embodiment, a storage 140 may include NAND flash memory142. In another embodiment, a storage 140 may include MRAM 150. In acertain embodiment, a storage 140 may include a combination of NANDflash memory 142, MRAM 150, and/or other storage media types.

In various embodiments, MRAM 150 may be used to store data in a cache120, memory 130, storage 140, and/or another component that stores data.For example, in the depicted embodiment, the computing device 110includes MRAM 150 in the cache 120, memory 130, and storage 140. Inanother embodiment, a computing device 110 may use MRAM 150 for memory130, and may use other types of memory or storage media for cache 120 orstorage 140. Conversely, in another embodiment, a computing device 110may use MRAM 150 for storage 140, and may use other types of memorymedia for cache 120 and memory 130. Additionally, some types ofcomputing device 110 may include memory 130 without storage 140 (e.g.,in a microcontroller) if the memory 130 is non-volatile, may includememory 130 without a cache 120 for specialized processors 115, or thelike. Various combinations of cache 120, memory 130, and/or storage 140,and uses of MRAM 150 for cache 120, memory 130, storage 140, and/orother applications will be clear in view of this disclosure.

In various embodiments, the MRAM 150 may include one or more chips,packages, die, or other integrated circuit devices comprisingmagnetoresistive memory, disposed on one or more printed circuit boards,storage housings, and/or other mechanical and/or electrical supportstructures. For example, one or more dual inline memory modules (DIMMs),one or more expansion cards and/or daughter cards, a solid-state-drive(SSD) or other storage device, and/or another memory and/or storage formfactor may comprise the MRAM 150. The MRAM 150 may be integrated withand/or mounted on a motherboard of the computing device 110, installedin a port and/or slot of the computing device 110, installed on adifferent computing device 110 and/or a dedicated storage appliance on anetwork, in communication with a computing device 110 over an externalbus, or the like.

The MRAM 150, in various embodiments, may include one or more MRAM die,including a plurality of magnetic tunnel junctions (MTJs) for storingdata. In certain embodiments, an MTJ includes a reference layer, abarrier layer, and a free layer. In further embodiments, a free layermay include a nucleation region and one or more arms. A nucleationregion may be configured to form a magnetic domain wall, and an arm mayinclude a plurality of pinning sites formed at predetermined locationsalong the arm for pinning the domain wall. In certain embodiments, anMTJ that includes a nucleation region and one or more arms with pinningsites may provide intermediate resistance states between a lowresistance or parallel state and a high resistance or antiparallelstate. In further embodiments, an MTJ or magnetic memory cell thatprovides more than two resistance states, or magnetization states, maybe used to store more than one bit of data. For example, an MTJ thatprovides four resistance states may store two bits of data, so that thefour states correspond to binary numbers 00, 01, 10, and 11. Similarly,an MTJ that provides eight resistance states may store three bits ofdata; an MTJ that provides sixteen resistance states may store four bitsof data, and so on. MRAM 150 is described in further detail below withregard to FIGS. 2 through 10.

FIG. 2 depicts one embodiment of an MRAM die 150. The MRAM die 150 maybe substantially similar to the MRAM 150 described with reference toFIG. 1. The MRAM die 150, in the depicted embodiment, includes an array200 of MRAM cells, row circuits 202, column circuits 204, and a diecontroller 206.

In various embodiments, an MRAM die 150 may be an integrated circuitthat includes both a core array 200 of memory cells (e.g., MTJs) formagnetoresistive data storage, and peripheral components (e.g., rowcircuits 202, column circuits 204, and/or die controller 206) forcommunicating with the array 200. In certain embodiments, one or moreMRAM die 150 may be included in a memory module, a storage device, orthe like.

In the depicted embodiment, the array 200 includes a plurality of memorycells (e.g., MRAM cells, MTJs, or the like). In one embodiment, thearray 200 may be a two-dimensional array. In another embodiment, thearray 200 may be a three-dimensional array that includes multiple planesand/or layers of MRAM cells. In various embodiments, the array 200 maybe addressable by rows via row circuits 202, and by columns via columncircuits 204.

The die controller 206, in certain embodiments, cooperates with the rowcircuits 202 and the column circuits 204 to perform memory operations onthe array 200. In various embodiments, the die controller 206 mayinclude components such as a power control circuit that controls thepower and voltages supplied to the row circuits 202 and column circuits204 during memory operations, an address decoder that translates areceived address to a hardware address used by the row circuits 202 andcolumn circuits 204, a state machine that implements and controls thememory operations, and the like. The die controller 206 may communicatewith a computing device 110, a processor 115, a bus controller, astorage device controller, a memory module controller, or the like, viabus 208, to receive command and address information, transfer data, orthe like.

FIG. 3 depicts a portion 300 of an artificial neural network, in oneembodiment. In the depicted embodiment, a portion 300 of an artificialneural network includes an artificial neuron 306, and a plurality ofsynapses 304. Arrows in FIG. 3 represent the flow of information, frominputs 302 on the left, to an output 312 on the right.

In various embodiments, an artificial neuron 306 may be analogous to abiological neuron. A biological neuron may include a plurality ofdendrites that receive excitatory or inhibitory signals across synapses,weighted by synaptic neurotransmitters, a soma where ions correspondingto the weighted excitatory or inhibitory signals mix, and an axon thattransmits an output signal based on an electrical potential of the mixedions. Similarly, an artificial neuron 306 may produce a signal at anoutput 312 (analogous to a biological axon) based on a summation ofweighted inputs 302 (where the inputs are analogous to biologicaldendrites and the weighting is analogous to biological synapses).

In various embodiments, an artificial neural network may include aplurality of interconnected artificial neurons 306, analogous tointerconnected biological neurons in a brain or nervous system. Invarious embodiments, artificial neurons 306 may receive inputs 302 fromother artificial neurons 306 and/or from external sources. Similarly,the output 312 of an artificial neuron 306 may be provided to anexternal component, or may be provided to one or more further artificialneurons 306. Thus, the depicted portion 300, including an artificialneuron 306 and synapses 304 may be repeated many times in an artificialneural network.

In the depicted embodiment, the inputs 302 may receive binary or analogsignals x₁ through x_(n) from other artificial neurons 306, or fromsources external to an artificial network. For example, in oneembodiment, an input 302 may receive a signal to be processed byartificial neural network. In a further embodiment, an input 302 mayreceive a signal from another artificial neuron 306. In a certainembodiment, an input 302 may be a bias input that is set to a fixed biaslevel (or to 1, so that the bias level is controlled by a synapse 304.Inputs 302 may be electrical lines, optical lines, or any other hardwarecapable of conveying the input signals x₁ through x_(n).

Synapses 304, in various embodiments, provide weighted input signals tothe artificial neuron 306. For example, the synapses 304 may provide aplurality of weights w₁ through w_(n) that are multiplied bycorresponding input signals x₁ through x_(n), so that the artificialneuron 306 receives a first weighted input w₁x₁, a second weighted inputw₂x₂, and so on. In one embodiment, synapses 304 may include latches,registers, SRAM memory cells, volatile memory cells, non-volatile memorycells, or any other hardware capable of storing weights w₁ throughw_(n). In one embodiment, a synapse 304 may further include logichardware for multiplying the input signals x₁ through x_(n) by theweights w₁ through w_(n). In another embodiment, the synapses 304 maystore the weights, and the multiplication may be performed by logichardware separate from the synapses 304, such as a processor 115, anFPGA, an ASIC, a state machine, or the like. In another embodiment, theinputs x₁ through x_(n) may be binary inputs, or may be converted tobinary by comparison to a threshold, and a synapse 304 for input x_(k)may output a weight w_(k) (e.g., the weight w_(k) may be read from asynapse 304 storage location) if the input is 1, or may output a 0(e.g., a read operation for synapse 304 storage locations does not readfrom that synapse 304) if the input is zero. Various further types andconfigurations of hardware for synapses 304 will be clear in view ofthis disclosure.

In the depicted embodiment, the artificial neuron 306 includes asummation component 308. The summation component 308, in variousembodiments, may receive and sum the weighted input signals w₁x₁ throughw_(n)x_(n). The summation component 308 may include an adder, or anyother hardware capable of summing inputs, or may be implemented by aprocessor 115 executing code. In a further embodiment, the artificialneuron 306 includes an activation component 310 that produces a signalat the output 312 based on the summed weighted input signals. Theactivation component 310 may output a function of the summed weightedinput signals, such as a step function (for a perceptron), a sigmoidfunction (for a sigmoid neuron), a rectified linear function (for a ReLUneuron), or another non-linear function. In certain embodiments, theactivation component 310 may produce an output signal as a function ofthe summed weighted input signals and of time. For example, theactivation component 310 may output a spike that decays back to zero.The activation component 310 may include a comparator or other logichardware to produce the output signal, or may be implemented by aprocessor 115 executing code. Various types and configurations ofhardware for summation components 308 and activation components 310 willbe clear in view of this disclosure.

In certain embodiments, an artificial neural network may include manyportions similar to the depicted portion 300. For example, an artificialneural network may include a plurality of artificial neurons 306, andsynapses 304 that store weights for couplings between artificial neurons306. For example, in one embodiment, an artificial neural network mayinclude a number N of artificial neurons 306, and an array of N*Nsynapses 304 that store weights for couplings between artificial neurons306. A weight of zero may be equivalent to two corresponding artificialneurons 306 not being coupled together. A neuromorphic computing die mayinclude artificial neurons 306 and an array of synapses 304.

FIG. 4 depicts one embodiment of a neuromorphic computing die 450. Theneuromorphic computing die 450, in the depicted embodiment, includes asynapse array 400, row/axon circuits 402, column/dendrite circuits 404,a plurality of artificial neurons 410, and a die controller 206.

In various embodiments, the neuromorphic computing die 450 may be anintegrated circuit that includes artificial neurons 410 and a synapsearray 400, and peripheral circuits (e.g., row/axon circuits 402,column/dendrite circuits 404, and/or die controller 206) for computingusing the artificial neurons 410. In various embodiments, one or moreneuromorphic computing dies 450 may be included in a computing device110, and may be used in place of, or in addition to, a processor 115.

A plurality of artificial neurons 410, in certain embodiments, may besubstantially similar to the artificial neurons 306 described above withreference to FIG. 3, including summation components 308 and activationcomponents 310. Similarly, the synapse array 400 may be an array ofsynapses 304, substantially as described above with reference to FIG. 3.The synapses 304 may store weights for connections between neurons 410.For example, a synapse 304 in row i and column j of the array 400 maystore a weight that couples the output 312 (or axon) of neuron i to aninput 302 (or dendrite) of neuron j. Thus, rows of the synapse array 400may correspond to outputs 312 or axons, and columns of the array 400 maycorrespond to inputs 302 or dendrites. The synapse array 400 may beaddressable by rows via row/axon circuits 402, and by columns viacolumn/dendrite circuits 404. In one embodiment, the row/axon circuits402 and column/dendrite circuits 404 may be substantially similar to therow circuits 202 and column circuits 204 of the MRAM die 150 describedabove with reference to FIG. 2. In a further embodiment, the synapsearray 400, row/axon circuits 402, and column/dendrite circuits 404 maybe transposable, so that a row of weights or a column of weights may beread or written together.

The die controller 206, may be substantially similar to the diecontroller 206 described above with reference to FIG. 2, and maycommunicate with a computing device 110, a processor 115, a buscontroller, or the like, via bus 208, to receive commands and transferdata.

In various embodiments, the neuromorphic computing die 450 may computeby receiving inputs for the artificial neurons 410 via the bus 208,processing the inputs through a network of neurons 410 interconnected bysynapses 304 of the synapse array 400 (as described above with referenceto FIG. 3), and providing outputs via the bus 208. The die controller206 may receive the inputs, provide the outputs, and may control therow/axon circuits 402 and column/dendrite circuits 404 to access ormodify weights stored by the synapse array 400. The die controller 206may modify weights according to a learning algorithm, or may useexisting weights for computation.

As used herein, the term “neuromorphic” may refer to artificial neurons410, to components associated with artificial neurons 410, such assynapses 304, or a synapse array 400, and to systems, apparatuses, ormethods that include or use artificial neurons 410 for computation.Thus, referring to a computing die 450 as “neuromorphic” may indicatethat the neuromorphic computing die uses an artificial neural networkfor computing instead of, or in addition to, logic hardware thatexecutes instructions. Similarly, a “neuromorphic” computing array mayinclude artificial neurons 410 and synapses 304 coupling the neurons,whether on a neuromorphic computing die 150, or as part of anothercomputing device.

In general, in various embodiments, a neuromorphic computing device maycompute by processing signals through a network of interconnectedartificial neurons 410. Thus, the transformation of input signals tooutput signals may be based on the type and number of neurons 410, theconnections between those neurons 410, and the synaptic weights storedin the synapse array 400. A neuromorphic computing device may beprogrammed by setting synaptic weights. In certain embodiments, a diecontroller 206 may apply a learning algorithm to iteratively set andadjust synaptic weights in the synapse array 400.

In certain embodiments, a synapse array 400 may be an array of storagecells or MRAM cells substantially similar to a storage array (e.g., theMRAM array 200 of FIG. 2). A synapse 304 may be a storage cell or set ofstorage cells that stores a synaptic weight, and may be said to “couple”two artificial neurons 306 if the output signal of one artificial neuron306 is multiplied by the stored synaptic weight and provided as an inputsignal to another artificial neuron 306. Thus, a synapse 304 may bereferred to as “coupling” two artificial neurons 306 based on a physicalconfiguration that physically couples the output 312 of one artificialneuron 306 to an input 302 of another artificial neuron 306 via thesynapse 304, or based on a logical configuration that references thesynapse 304 when propagating an output signal of one artificial neuron306 as an input signal to another artificial neuron 306. For example, adie controller 206 may read a synaptic weight from a synapse 304 at rowi and column j of the synapse array 400, multiply the synaptic weight bythe output signal of neuron i, and provide the weighted signal as aninput signal to neuron j. The synapse 304 at row i and column j may thenbe referred to as “coupling” neuron i and neuron j, regardless ofwhether the synapse 304 is physically coupled to those artificialneurons 306.

A multi-weight synapse 304, in various embodiments, may be a synapse 304that stores or represents multiple intermediate weight states between alowest weight state and a highest weight state. Increasing the number ofpossible states, or possible synaptic weights per synapse 304 mayincrease computing capacity, robustness, and precision, for a givennumber of artificial neurons 306. In one embodiment, a multi-weightsynapse 304 may include a plurality of single-bit storage cells, such asSRAM cells, SLC Flash cells, or the like, which each store one bit,either a 1 or a 0. In another embodiment, however, a multi-weightsynapse 304 may include one or more multi-state memory cells. Althougheven a single-bit cell provides two states, corresponding to a 1 or a 0,the term “multi-state” cell is used herein to refer to a cell thatprovides more than two states, and that stores more than one bit percell. For example a multi-state cell may provide four states for storingtwo bits, eight states for storing three bits, sixteen states forstoring four bits, or the like. For the same precision, a synapse array400 that stores two bits per cell may be half the size or area of asynapse array 400 that stores one bit per cell.

In various embodiments, feed-forward classification for an artificialneural network involves reading and applying synaptic weights, and isfacilitated by synapses 304 that can be read quickly, with low readenergy, and with low read disturbance (e.g., without changing weightsstored by other synapses 304). In further embodiments, back-propagatedlearning for an artificial neural network involves changing synapticweights, and is facilitated by synapses 304 that provide fast writeoperations with low write energy, and with high endurance. MRAM memorycells based on magnetic tunnel junctions may provide fast, low-energyread and write operations, with high retention, high endurance, and lowread disturbance. However, a synapse array 400 of two-state (paralleland antiparallel) magnetic tunnel junctions may use multiple cells torepresent a multi-weight synapse 304, and may be large compared to anarray of multi-state cells. Instead, in certain embodiments, a synapsearray 400 may include multi-state magnetic memory cells (e.g., MTJs), asdescribed with reference to subsequent Figures, as synapses 304, toprovide the same precision as an array of two-state cells, with lessarea. In certain embodiments, multi-state MTJs may provide higherendurance than certain other types of multi-state resistive memorycells.

FIG. 5 depicts one embodiment of a magnetic tunnel junction 500. The MTJ500, in certain embodiments, may be a magnetic memory cell, or may bepart of a magnetic memory cell. A memory cell, in various embodiments,may be the smallest data-storing structure of a memory or storage mediumor array, such as the MRAM array 200 or the synapse array 400, and mayhave a physical or electrical property that is configured to bemodified, to store data. A range of possible values for the data-storingphysical property may be divided into states, corresponding to datavalues. In further embodiments, a magnetic memory cell may be a memorycell that uses a magnetic physical property, such as a magnetic moment,a magnetization, or the like, to store data. In certain embodiments, amagnetic memory cell may include an MTJ 500, which may store data basedon the magnetization or magnetic moment of a free layer 502 relative toa reference layer 506. In another embodiment, a magnetic memory cell maybe a cell other than an MTJ 500, which uses magnetization to store data.

The MTJ 500, in the depicted embodiment, includes a free layer 502, abarrier layer 504, and a reference layer 506. Two terminals 522, 524 aredepicted for convenience in discussing current flows. In certainembodiments, an MTJ 500 may include or be coupled to metallic or otherconductive terminals 522, 524 for coupling the MTJ 500 to lines of anMRAM array 200. In another embodiment, however, lines of an array 200may be directly coupled to the MTJ 500.

In various embodiments, layers of the MTJ 500 may be formed or depositedby various techniques such as physical vapor deposition, sputtering, orthe like. In certain embodiments, further layers not shown in FIG. 5,such as pinning layers, pinned layers, capping layers, or seed layers,may be included in an MTJ 500, in a memory cell that includes an MTJ500, or in the process of making an MTJ 500 or memory cell.

An MTJ 500, in the depicted embodiment, includes a fixed or referencelayer 506 with a fixed or pinned magnetic moment, indicated by asingle-headed arrow. In a further embodiment, an MTJ 500 includes astorage or free layer 502, with a magnetic moment that can be changed orswitched, indicated by a double-headed arrow. A thin dielectric orbarrier layer 504 may separate the reference layer 506 from the freelayer 502, and current may flow across the barrier layer 504 due toquantum tunneling. The probability of an electron tunneling through thebarrier layer 504 is higher if the magnetic moments of the referencelayer 506 and the free layer 502 are substantially parallel to eachother (referred to herein as the parallel state for the MTJ 500), andlower if the magnetic moments of the reference layer 506 and the freelayer 502 are substantially antiparallel to each other (referred toherein as the antiparallel state for the MTJ 500). Therefore, anelectrical resistance through the MTJ 500 may be higher in theantiparallel state than in the parallel state.

In various embodiments, a difference in resistance between parallel andantiparallel states of an MTJ 500 allows data to be stored. For example,a low resistance may correspond to a binary “1” and a high resistancemay correspond to a binary “0,” Alternatively, a low resistance maycorrespond to a binary “0” and a high resistance may correspond to abinary “1.”

In certain embodiments, a magnetic storage or free layer 502 may bemagnetized as a single domain, which may be magnetized parallel orantiparallel to the reference layer 506, corresponding to a lowresistance state and a high resistance state for the MTJ 500. In furtherembodiments, a storage or free layer 502 may include two or moremagnetic domains, with one or more domain walls between domains in thefree layer 502, so that at least one domain is magnetized parallel tothe reference layer 506 and at least one domain is magnetizedantiparallel to the reference layer 506. In certain embodiments, acontroller such as a die controller 206 may apply a write current thatforms, moves, or removes a domain wall from the free layer 502. Aresistance through the MTJ 500 when the free layer 502 includes multipledomains may correspond to a ratio between parallel-magnetized volume ofthe free layer 502 and antiparallel-magnetized volume of the free layer502. Thus, a partially parallel and partially antiparallel free layer502 may provide one or more intermediate resistance states for an MTJ500, between a low resistance state and a high resistance state. Forexample, in one embodiment, four resistance states may correspond (inorder from highest to lowest resistance) to antiparallel, mostlyantiparallel, mostly parallel, and parallel states for the free layer502. An MTJ 500 that provides intermediate resistance states may storeor encode more than one bit. For example, an MTJ 500 that provides fourstates (e.g., two intermediate states between high and low resistancestates) may encode two bits, so that the binary values 00, 01, 10, and11 each correspond to one of the four states.

The fixed or reference layer 506, in one embodiment, includes aferromagnetic material with a fixed or pinned magnetic moment. As usedherein, the term “ferromagnetic” may be used to refer to any materialcapable of spontaneous magnetization (e.g., remaining magnetized in theabsence of an externally applied magnetic field). Thus, a“ferromagnetic” material may refer to a strictly ferromagnetic material(e.g., for which individual microscopic magnetic moments are fullyaligned), or to a ferrimagnetic material (e.g., for which individualmicroscopic magnetic moments are partially anti-aligned).

In various embodiments, a “fixed” or “pinned” magnetic moment refers toa magnetic moment that is substantially constant, at least inorientation, when the magnetic moment of the free layer 502 is changed.Thus, for example, in one embodiment, a reference layer 506 may comprisea ferromagnetic material with a higher coercivity than a ferromagneticmaterial of a free layer 502. In such an embodiment, an externalmagnetic field may change the magnetization of both the reference layer506 and the free layer 502, but with a larger effect on the free layer502. In another embodiment, a reference layer 506 may comprise aferromagnetic thin film with a magnetic moment pinned by exchangecoupling with an antiferromagnet. For example, in one embodiment, areference layer 506 may include a synthetic antiferromagnet (e.g., acobalt/iron and ruthenium multilayer), a ruthenium or iridium spacer,and a ferromagnetic layer comprising a cobalt/iron/boron alloy (CoFeB).

The magnetic moment of the reference layer 506, in various embodiments,may provide a reference for the orientation of one or more magneticmoments of the free layer 502. For example, in various embodiments, oneor more magnetic moments for domains of the free layer 502 may beparallel or antiparallel to the magnetic moment of the reference layer506. Thus, the reference layer 506 is depicted with a reference magneticmoment indicated by a single-headed arrow, and the parallel orantiparallel magnetic moment(s) of the free layer 502 are indicated by adouble-headed arrow.

In the depicted embodiment, the magnetic moments of the reference layer506 and the free layer 502 are in-plane magnetic moments. In anotherembodiment, however, the magnetic moments of the reference layer 506 andthe free layer 502 are in-plane magnetic moments. As used herein, termssuch as “in-plane” and “perpendicular” may be used to describe adirection or orientation (e.g., for a vector quantity such as a magneticmoment, magnetization, current density, or the like), relative to alayer of an MTJ 500. In one embodiment, the term “perpendicular” refersto a direction at right angles to a surface of a layer (e.g., verticallyin FIG. 5), and the term “in-plane” refers to a direction parallel to asurface of the layer (e.g., horizontally in FIG. 5). In anotherembodiment, however, a vector, orientation, or direction may include acombination of perpendicular and in-plane components, and but may bedescribed as either “perpendicular” or “in-plane” based on whether theperpendicular component or the in-plane component has a greatermagnitude. For example, in one embodiment, where a magnetic momentincludes non-zero in-plane and perpendicular components, it maynevertheless be described as an “in-plane” magnetic moment if thein-plane component is greater than the perpendicular component.

For example, in the depicted embodiment, with in-plane magnetic moments,a magnetic moment for the reference layer 506 may point toward the leftof FIG. 5, so that a parallel magnetic moment for a domain of the freelayer 502 points toward the left of FIG. 5, and an antiparallel magneticmoment for a domain of the free layer 502 points toward the right ofFIG. 5. In another embodiment, with magnetic moments perpendicular tothe layers, a magnetic moment for the reference layer 506 may pointtoward the top of FIG. 5, so that a parallel magnetic moment for adomain of the free layer 502 points toward the top of FIG. 5, and anantiparallel magnetic moment for a domain of the free layer 502 pointstoward the bottom of FIG. 5.

The barrier layer 504, in various embodiments, is disposed between thefixed or reference layer 506 and the free layer 502. In certainembodiments, the barrier layer 504 comprises a dielectric material, suchas magnesium oxide (MgO). In certain embodiments, the barrier layer 504may be less than 20 angstroms thick, so that quantum tunneling ofelectrons across the barrier layer 504 allows current to flow throughthe MTJ 500.

In general, in various embodiments, the free layer 502 may comprise aferromagnetic material, for which the magnetic moment of the free layer502 as a single domain, or the magnetic moments of one or more magneticdomains within the free layer 502, can be changed, switched, or flipped,relative to the magnetic moment of the reference layer 506. Changing themagnetic moment(s) of the free layer 502 changes the electricalresistance of the MTJ 500, allowing data to be stored. In certainembodiments, a ferromagnetic material of the free layer 502 may includea CoFeB alloy. In some embodiments, the free layer 502 may includemultilayers based on transition metals like cobalt and iron, and noblemetals such as platinum, palladium, and gold. Some examples includecobalt/palladium, cobalt/platinum and cobalt/nickel.

Reading data from an MTJ 500, in various embodiments, may includemeasuring, detecting, or sensing a resistance of the MTJ 500 (e.g.,indicating whether the MTJ 500 is in a parallel, antiparallel, orintermediate state). For example, in one embodiment, a known voltage maybe applied across a first terminal T1 522 and a second terminal T2 524,and the resulting current through the free layer 502, the barrier layer504, and the reference layer 506 may be measured or sensed to detect theresistance of the MTJ 500. In another embodiment, a known current may beapplied through the free layer 502, the barrier layer 504, and thereference layer 506, and the resulting voltage drop between the firstterminal T1 522 and the second terminal T2 524 may be measured or sensedto detect the resistance. In certain embodiments, an MRAM array 200,MRAM die 150, neuromorphic computing die 450, or the like may includesense amplifiers, latches, and the like, to convert a low power signalto a logic level representing a data value, and to store the converteddata.

Writing data to an MTJ 500, in various embodiments, may include settingor changing magnetization in the free layer 502 so that the MTJ 500 isin the desired parallel, intermediate, antiparallel state. Various typesof MRAM provide various ways to set the magnetic moment of the freelayer 502. In spin-transfer torque (STT) MRAM, data may be written bypassing a spin-polarized electrical current through an MTJ 500 (e.g.,between the first terminal T1 522 and the second terminal T2 524), tochange magnetization in the free layer 502. By contrast, in spin-orbittorque (SOT) MRAM, data may be written by applying an electrical currentthrough a spin Hall effect material adjacent to the free layer 502(e.g., from a first write terminal T1 522, parallel to the free layer502, to a second write terminal, not shown in FIG. 5), thus generating apure spin current for changing magnetization in the free layer 502. Inanother embodiment, data may be written by applying a magnetic field tothe free layer 502, and so that the applied field exerts a torque onmagnetic moment(s) of the free layer 502. For example, in oneembodiment, data may be written by applying an electrical currentthrough one or more write wires or write lines near or adjacent to thefree layer 502, so that the current produces a magnetic field around thewrite lines in accordance with Ampere's law.

In various embodiments, a controller, such as the die controller 206,may supply an electrical write current for writing data to the MTJ 500.In one embodiment, the write current may control a magnetic fieldapplied to the free layer 502. In another embodiment, the write currentmay control a spin current injected into the free layer 502, either inthe form of a spin-polarized electrical current for STT MRAM, or in theform of a pure spin current for SOT MRAM. For example, in oneembodiment, the magnitude of the applied magnetic field, or of theinjected spin current, may be proportional to the write current, or mayincrease with the write current. In a certain embodiment, increasing thewrite current may increase the applied magnetic field, or the injectedspin current. Thus, in one embodiment, a controller may use differentwrite currents to program an MTJ 500 to different resistance states. Inanother embodiment, reversing the write current may reverse thedirection of the applied magnetic field, or the injected spin current.Thus in one embodiment, a controller may apply write currents indifferent directions to increase or decrease the resistance of the MTJ500.

Thus, in one embodiment, a die controller 206 for a memory array such asthe MRAM array 200 may write or modify data by controlling writecurrents for memory cells or MTJs 500 in the array 200 via the rowcircuits 202 and the column circuits 204. Similarly, in anotherembodiment, a die controller 206 for a synapse array 400 may modifysynapse weights for the synapse array 400 by controlling write currentsfor multi-state magnetic memory cells (e.g., MTJs 500) of the array viathe row/axon circuits 402 and the column/dendrite circuits 404.

A controller may include or communicate with power generating componentssuch as current sources, voltage sources, level shifters or the like,switching components such as transistors for coupling currents to MTJs500, sensing components such as sense amplifiers, latches to store senseamplifier outputs, and the like.

FIG. 6A through FIG. 8 depict top views of various embodiments of freelayers 600, 700, 800, for a magnetic tunnel junction 500 or multi-statemagnetic memory cell. The free layers 600, 700, 800, in certainembodiments, may be substantially similar to the free layer 502described above with reference to FIG. 5. For convenience in depictingthe shape, outline, or configurations of the free layers 600, 700, 800,further layers of an MTJ 500 such as a barrier layer 504 or a referencelayer 506 are not shown in FIG. 6A through FIG. 8.

In various embodiments, a shaped free layer that provides multipleresistance states for an MTJ 500, such as the free layer 600 of FIGS.6A-6D, the free layer 700 of FIGS. 7A-7H, or the free layer 800 of FIG.8, may be formed by various techniques such as physical vapordeposition, sputtering, and may be shaped by using lithography tocontrol where ferromagnetic material is deposited, using e-beam millingto control where ferromagnetic material is removed, or the like. In oneembodiment, layers such as a barrier layer 504 or a reference layer 506may be shaped to match a free layer 600, 700, 800. For example, materialaddition and removal processes may be similar for the fixed or referencelayer 506, the barrier layer 504 and the free layer 600, 700, 800, sothat the reference layer 506, the barrier layer 504 and the free layer600, 700, 800 are similarly shaped. In another embodiment, layers suchas a barrier layer 504 or a reference layer 506 may be shaped to extendbeneath (or above) the free layer 600, 700, 800, but may also extendbeyond the edges of the free layer 600, 700, 800. For example, materialaddition processes may be similar for the fixed or reference layer 506,the barrier layer 504 and the free layer 600, 700, 800, but materialremoval processes may be carried out only for the free layer 600, 700,800, so that the barrier layer 504 and the reference layer 506 occupymore area than the free layer 600, 700, 800. As another example,lithographic masks may be simpler for the reference layer 506 and thebarrier layer 504 than for the free layer 600, 700, 800 so that arectangular, oval or other simply-shaped reference layer 506 and barrierlayer 504 extend beneath a free layer 600, 700, 800 with a morecomplicated shape. In an array 200, 400, the space between cells, andspace where ferromagnetic material is removed (or omitted) from the freelayer 600, 700, 800 may be left empty, may be filled with anon-ferromagnetic dielectric material to provide structural support forthe cells while avoiding short circuits between cells, or the like.

In FIGS. 6A-7H, arrows represent the magnetization of the free layers600, 700, or of domains within the free layers 600, 700. Additionally,in FIGS. 6A-7H, the magnetization of the fixed or reference layer 506 isto the right of the Figure, so that a left-pointing arrow represents anantiparallel state for a free layer 600, 700, or for a domain within thefree layers 600, 700, and so that a right-pointing arrow represents aparallel state for a free layer 600, 700, or for a domain within thefree layers 600, 700. However, the depicted and described magnetizationsare for exemplary purposes, and are not intended as limiting. Forexample, a magnetization for a reference layer 506 may be in anotherin-plane or perpendicular direction, with corresponding parallel andantiparallel directions in free layers 600, 700.

FIGS. 6A-6D depict different resistance states for a free layer 600 of amagnetic tunnel junction 500 or multi-state magnetic memory cell, in oneembodiment. In the depicted embodiment, the free layer 600 includes anucleation region 602, a wall extension region 604, and a plurality ofpinning sites 606, 608. FIGS. 6A through 6D depict resistance states indescending order from a high resistance state in FIG. 6A to a lowerresistance state in FIG. 6D.

In certain embodiments, the free layer 600 may provide a plurality ofresistance states for an MTJ 500: a high resistance state, where thefree layer 600 is magnetized antiparallel to the reference layer 506; alow resistance state, where the free layer 600 is magnetized parallel tothe reference layer 506; and a plurality of intermediate resistancestates where the free layer 600 includes at least one magnetic domainmagnetized parallel to the reference layer 506, at least one magneticdomain magnetized antiparallel to the reference layer 506, and at leastone domain wall 650 separating parallel and antiparallel domains. In afurther embodiment, the plurality of intermediate resistance states maycorrespond to positions of the domain wall 650 within the free layer 600(e.g., within the wall extension region 604). In certain embodiments, ifa free layer 600 is in a fully parallel or fully antiparallel state,without a domain wall 650, a write current or erase current from acontroller may cause a magnetic domain to form or nucleate in thenucleation region 602. As the write or erase current continues, orincreases, a domain wall 650 for the newly-formed domain may migrate asthe domain expands to saturate the nucleation region 602, leaving adomain wall 650 in the wall extension region 604. A first pinning site606 may pin the domain wall 650 at a predetermined location in the freelayer 600, providing a consistent intermediate resistance state.Additional energy (e.g., from an increased write current) may unpin thedomain wall 650 from a first pinning site 606, and move the domain wall650 to a second pinning site 608, corresponding to a second intermediateresistance state.

In FIG. 6A, the free layer 600 is magnetized as a single domain,antiparallel to the magnetic moment of the reference layer 506. Adomain, or magnetic domain, in various embodiments, may be a region ofuniform or substantially uniform magnetization for a magnetic material.Within a domain, individual microscopic magnetic moments may be alignedfor strictly ferromagnetic materials, or partially anti-aligned forferrimagnetic materials, resulting in a net magnetic moment for thedomain. A domain wall 650 (present in FIGS. 6B and 6C but not in FIG. 6Aof 6D), in various embodiments, may be a boundary region betweendifferent domains of a magnetic material. Individual microscopicmagnetic moments may transition across a domain wall 650, from beingaligned with a magnetic moment for a first domain, to being aligned witha magnetic moment for a second domain on the opposite side of the domainwall 650. Within the domain wall 650, between domains with two differentmagnetic moments, individual microscopic magnetic moments may be alignedwith neither magnetic moment, but may point in intermediate directionsbetween the first magnetic moment and the second magnetic moment.Various types of domains, and domain walls 650, such as Bloch walls,Neel walls, and hybrid walls, will be clear in view of this disclosure.

Terms such as “parallel” and “antiparallel” may be used herein to referto an orientation for the magnetic moment or magnetization of the freelayer 600, 700, 800, or of a domain within the free layer 600, 700, 800,relative to the magnetic moment or magnetization of the reference layer506. Thus, a “parallel” magnetic moment or magnetization for a domain,or for the free layer 600, 700, 800, refers to a magnetic moment ormagnetization parallel to the magnetic moment of the reference layer506, even if the magnetic moment of the reference layer 506 is notexplicitly referred to. Similarly, the free layer 600, 700, 800, or adomain within the free layer 600, 700, 800 may be referred to as“parallel” or “antiparallel” based on a corresponding parallel orantiparallel magnetization, without explicit reference to the magneticmoment of the reference layer 506.

In FIG. 6A, the single-domain free layer 600 has an antiparallelmagnetic moment, corresponding to a high resistance state for the MTJ500. In certain embodiments, a write current from a controller, such asthe die controller 206, may control a magnetic field applied to the freelayer 600, or a spin current injected into the free layer 600, and a newdomain, and domain wall 650, may form in the nucleation region 602 inresponse to the write current.

The nucleation region 602, in certain embodiments, may be a region ofthe free layer 600 that is configured to form a domain wall 650, or tofacilitate formation of a domain wall 650. In certain embodiments, thenucleation region 602 may be larger, in at least one dimension, than awall extension region 604. For example, in the depicted embodiment, thewall extension region 604 is an elongate arm or track, and thenucleation region 602 is a circular region with a diameter approximatelythree times the width of the elongate arm. In certain embodiments, theincreased dimension of the nucleation region 602, relative to adimension of the wall extension region 604 may facilitate formation of adomain wall 650 in the nucleation region 602 rather than in the wallextension region 604.

In certain embodiments, the nucleation region 602 may be a pad-shapedregion of the free layer 600. An area, such as a region of a free layer600 may be referred to as “elongate” if a length (or longest dimension)of the region is substantially greater than the width (or shortestdimension) of the region (e.g., five times longer, ten times longer,twenty times longer, or the like), or as “pad-shaped” if a length (orlongest dimension) of the region is comparable to the width (or shortestdimension) of the region (e.g., equal to the width, less than double thewidth, less than triple the width, less than quadruple the width or thelike). In certain embodiments, a pad-shaped region may be convex. Forexample, a pad-shaped region may be a circle, oval, triangle, square,rectangle, rounded rectangle, oblong, pentagon, hexagon, or the like.Various shapes and sizes of pad-shaped regions suitable for use as anucleation region 602 of a free layer 600 will be clear in view of thisdisclosure.

The wall extension region 604, in various embodiments, may be a regionof the free layer 600 that is configured to provide a plurality ofresistance states for the memory cell or MTJ 500, where the resistancestates corresponding to positions of a domain wall 650 within the wallextension region 604. In certain embodiments, a wall extension region604 may be an arm or track extending from the nucleation region 602(e.g., horizontally, or within the plane of the free layer 600). An armor track may be an elongate region as described above. When referring toan elongate arm or track, terms such as “long,” “along the arm,”“length,” or the like are used herein to refer to directions towards oraway from the nucleation region 602, whereas terms such as “narrow,”“across the arm,” “width,” or the like are used to refer to directionsneither towards nor away from the nucleation region 602 (e.g., in theplane of the free layer 600, but at right angles to the length of thearm). In a certain embodiment, an arm or track extending from thenucleation region 602 as a wall extension region 604 may be narrowerthan the nucleation region 602, thus facilitating domain wall nucleationin the nucleation region 602 rather than in the arm. For example, awidth of an arm, relative to size of a nucleation region 602 (in thesame cross-arm) direction, may be three fourths the size of a nucleationregion 602, two thirds the size of the nucleation region 602, half thesize of the nucleation region 602, one third the size of the nucleationregion 602, one fourth the size of the nucleation region 602, or thelike.

In various embodiments, a domain wall 650 may form in the nucleationregion 602 and migrate to a position within the wall extension region604. In certain embodiments, the resistance of the MTJ 500 may be basedon a ratio of volumes for parallel and antiparallel domains of the freelayer 600. In a further embodiment, with a domain wall 650 in anelongate wall extension region 604, the parallel to antiparallel volumeratio may substantially correspond to the position of the domain wall650 along the length of the wall extension region 604, rather than tothe shape of the domain wall 650 across the wall extension region 604.Thus, in certain embodiments, an elongate wall extension region 604 mayprovide a plurality of resistance states for the MTJ 500, correspondingto positions of a domain wall 650 along the wall extension region 604.

In certain embodiments, a free layer 600 may include one or more wallextension regions 604, or arms, extending from a nucleation region 602or pad-shaped region. Multiple arms or wall extension regions 604 arediscussed in further detail below with reference to FIGS. 7A-8.

A plurality of pinning sites 606, 608, in certain embodiments, may beconfigured to pin the domain wall 650. In a further embodiment, a wallextension region 604 or arm may include a plurality of pinning sites606, 608 formed at predetermined locations along the wall extensionregion 604 or arm, for pinning a domain wall 650.

In various embodiments, as an applied field or an injected spin currentexpands a domain, moving a domain wall 650, the domain wall 650 may be“pinned” at a local energy minimum caused by a non-magnetic inclusion ina ferromagnetic region, a crystallographic defect or irregularity in thevolume of a ferromagnetic region, a defect or irregularity at an edge orsurface of a ferromagnetic region, or the like. Increasing the appliedfield or an injected spin current (e.g., by increasing a write currentfrom a controller) may cause the domain wall 650 to “unpin” from lonelocation and jump to a neighboring location. Thus, a domain may grow insudden steps as a domain wall 650 pins and unpins, rather than growingsmoothly. In certain materials, a domain wall 650 may pin to randomirregularities or defects in, or at the surface of a ferromagneticregion, such as a free layer 600 for an MTJ 500. A “pinning strength”for a location as used herein, may refer to any measurement, indication,or number corresponding to the difficulty of unpinning the domain wall650 from a location, such as an amount of energy sufficient to move thedomain wall 650, a write current sufficient to move the domain wall 650,a magnitude of an applied field sufficient to move the domain wall 650,or the like.

Although domain walls 650 may pin with low pinning strength to a varietyof random inclusions or irregularities, a pinning site 606, 608, invarious embodiments, may be a location configured to pin the domain wall650 with a significantly higher pinning strength than a random surfacedefect.

For example, a pinning site 606, 608, in certain embodiments, may be anirregularity or discontinuity in an edge, surface, or volume of the freelayer 600, which may be intentionally formed to be larger than randomlyformed defects, so as to have a greater pinning strength than randomlyformed defects. In various embodiments, a pinning site 606, 608, may beany structure intentionally formed in a free layer 600 of an MTJ 500,(e.g., in a wall extension region 604 or arm of the free layer 600) forpinning a domain wall 650. In the depicted embodiment, the pinning sites606, 608 are v-shaped notches that extend partway across the width ofthe wall extension region 604. In another embodiment, a pinning site606, 608 may be an indentation, a rounded, semicircular, or u-shapednotch, a rectangular notch, an indentation in the height of the wallextension region 604, a hole milled or formed through the wall extensionregion 604 from top to bottom, a protuberance increasing the width orheight of the wall extension region 604 at a particular location, a voidor gap decreasing the width or height of the wall extension region 604at a particular location, or the like. Various ways of forming pinningsites 606, 608 for a free layer 600 will be clear in view of thisdisclosure.

Locations for pinning sites 606, 608 in a free layer 600 (e.g., along awall extension region 604 or arm) may be predetermined by a manufacturerof an MTJ 500. A manufacturer may select locations for pinning sites606, 608, based on one or more factors such as a desired number ofresistance states for the MTJ 500, desired ratios or differences betweenresistances for different states, or the like. Various ways to determinelocations for pinning sites 606, 608 will be clear in view of thisdisclosure.

In certain embodiments, a domain wall 650 may migrate by unpinning fromone pinning site 606, and pinning to a subsequent pinning site 608, inresponse to an increased write current from a controller. In certainembodiments, pinning sites 606, 608 may be configured so that a domainformed in the nucleation region 602 expands in steps, as a domain wall650 migrates to a first pinning site 606, then to a second pinning site608, in order. A “subsequent” pinning site, as used herein, may refer tothe next pinning site (e.g., pinning site 608) encountered by amigrating domain wall 650 after unpinning from another pinning site(e.g., pinning site 606).

FIGS. 6A through 6D illustrate a free layer 600 at different times orresistance states, from prior to formation of a new domain or domainwall 650, to migration of the domain wall 650 to pinning sites 606, 608in sequence, to saturation of the free layer 600 by the newly formeddomain.

In FIG. 6A, the free layer 600 is a single domain in an antiparallelstate, so the MTJ 500 is in a high resistance state. A controller maychange a data value stored by the MTJ 500 by forming aparallel-magnetized domain within the free layer 600. A domain, and acorresponding domain wall 650, may form in or at the nucleation region602 in response to a write current from a controller.

In FIG. 6B, the newly-formed domain has expanded in response to thewrite current, saturating the nucleation region 602, and expanding untila domain wall 650 is pinned at the first pinning site 606. Thus, thenucleation region 602 and the wall extension region 604 out to the firstpinning site 606 are now magnetized parallel to the reference layer 506.The remainder of the wall extension region 604 remains magnetizedantiparallel to the reference layer 506, and the free layer 600 includestwo different domains. The volume ratio between the parallel andantiparallel domains determines the resistance of the MTJ 500. Becausethe fraction of parallel-magnetized volume has increased relative to thehigh resistance state of FIG. 6A, the resistance of the MTJ 500 hasdecreased in FIG. 6B.

In FIG. 6C, the parallel-magnetized domain has expanded in response toan increased write current from the controller. The increased writecurrent has unpinned the domain wall 650 from the first pinning site606, and expanded the parallel-magnetized domain to the second pinningsite 608. Thus, the nucleation region 602 and the wall extension region604 out to the second pinning site 608 are now magnetized parallel tothe reference layer 506, and the remainder of the wall extension region604 beyond the second pinning site 608 remains magnetized antiparallelto the reference layer 506. Because the fraction of parallel-magnetizedvolume has increased relative to the state depicted in FIG. 6B, theresistance of the MTJ 500 has further decreased in FIG. 4C.

In FIG. 6D, the parallel-magnetized domain has further expanded inresponse to an increased write current from the controller. Theincreased write current has unpinned the domain wall 650 from the secondpinning site 606, and expanded the parallel-magnetized domain tosaturate the wall extension region 604. The free layer 600 is thus asingle domain again, without a domain wall 650, fully magnetizedparallel to the reference layer 506, and the MTJ 500 is thus in a lowresistance state.

In various embodiments, such as the depicted embodiment of FIGS. 6A-6D,a resistance state for an MTJ 500 is based on whether a domain wall 650exists in the free layer 600, and where the domain wall 650 is pinned.Without a domain wall 650, the free layer 600 is magnetized fullyparallel or antiparallel to the reference layer 506, and the MTJ 500 isthus in a low resistance or high resistance state. If multiple domainsexist in the free layer 600, separated by a domain wall 650, the freelayer 600 is magnetized partially parallel and partially antiparallel tothe reference layer 506, and the position where the domain wall 650 ispinned determines the ratio of parallel to antiparallel volumes, andthus determines the resistance for the MTJ 500 in an intermediateresistance state between the high and low resistance states.

In a certain embodiment, the MTJ 500 may be configured to provide fouror more possible resistance states. For example, the free layer 600depicted in FIGS. 6A-6D provides four different resistance states. Inanother embodiment, a free layer 600 may include additional wallextension regions 604 extending from the nucleation region 602, and/oradditional pinning sites 606, 608, and thus may provide more than fourresistance states corresponding to different volume fractions forparallel versus antiparallel domains. For example, a newly-formed domainmay saturate a central nucleation region 602, and expand into multiplewall extension regions 604, forming multiple domain walls 650, andadditional resistance states may correspond to positions of a seconddomain wall 650 within a second wall extension region 604.

FIGS. 7A-7H depict another embodiment of a free layer 700 for a magnetictunnel junction 500, in a top view. In the depicted embodiment, the freelayer 700 includes a pad-shaped region 702, two arms 704, and aplurality of pinning sites 706, 708, 710 formed in the arms 704.

The pad-shaped region 702, in various embodiments, may be substantiallysimilar to the nucleation region 602 described above with reference toFIGS. 6A-6D, and may be configured to facilitate formation of a magneticdomain wall 750 in response to a write current. The arms 704 and pinningsites 706, 708, 710 in various embodiments, may be substantially similarto the wall extension region 604 and pinning sites 606, 608 describedabove with reference to FIGS. 6A-6D. A domain wall 750 may also besubstantially similar to the domain wall 650 described above withreference to FIGS. 6A-6D.

In the depicted embodiment, the pinning sites 706, 708, 710 are notches,and a notch depth, or a distance that the notch extends across an arm704 determines the pinning strength of the notches. In variousembodiments, deeper notches may have a greater pinning strength thanshallower notches.

In one embodiment, the pad-shaped region 702 may be a dot or disk,central to the free layer 700, and approximately 300 nanometers (nm) indiameter. For example, in one embodiment, the central dot or pad-shapedregion 702 may have a diameter in a range from 200 nm to 400 nm. In afurther embodiment, the central dot or pad-shaped region 702 may have adiameter in a range from 250 nm to 350 nm. In a certain embodiment, thecentral dot or pad-shaped region 702 may have a diameter in a range from290 nm to 310 nm.

The arms 704, in a certain embodiment, may be approximately 1000 nm long(e.g., in a range from 900 nm to 1100 nm, in a range from 950 nm to 1050nm, or the like), and may be approximately 100 nm wide (e.g., in a rangefrom 80 nm to 120 nm, in a range from 90 nm to 110 nm, in a range from95 nm to 105 nm, or the like). In various embodiments, notches forpinning sites 706, 708, 710, may have notch depths of approximately 20nm, approximately 40 nm, approximately 60 nm, or the like. In anotherembodiment, a free layer 700 may be scaled up or down proportionally, sothat an arm length is approximately ten times an arm width, a centraldot or pad-shaped region 702 diameter is approximately three times thearm width, and a notch depth is approximately 20%, 40%, or 60% of thearm width. Various possible sizes for a pad-shaped region 702 or centraldot, arms 704, and notches for pinning sites 706, 708, 710 will be clearin view of this disclosure.

In certain embodiments, a first arm 704 a includes one or moreintermediate pinning sites 706, of a first pinning strength, and aterminal pinning site 708 of a second pinning strength greater than thefirst pinning strength. Terms such as “intermediate,” and “terminal” maybe used herein with reference to pinning sites 706, 708, referring to adistance of the pinning site 706, 708 along the arm 704 from thepad-shaped region 702 (or from a similar nucleation region 602). Thus, a“terminal” pinning site 708 is a pinning site in an arm 704 furthestfrom the nucleation or pad-shaped region 702, and “intermediate” pinningsites 706 are disposed at intermediate positions along the arm 704between the nucleation or pad-shaped region 702 and a terminal pinningsite 708. In certain arms 704, such as a second arm 704 b, an “initial”pinning site 710 may further refer to a pinning site 710 of an arm 704that is closest to the nucleation or pad-shaped region 702, but that isconfigured differently from the intermediate pinning sites 706.

In certain embodiments, configuring a first arm 704 a with a terminalpinning site 708 with a greater pinning strength than the intermediatepinning site(s) 706 may prevent a domain wall 750 a from forming at, ordepinning from, the end of the first arm 704 a, while moving a domainwall 750 b in a second arm 704 b. Similarly, in the second arm 704 b, aninitial “arm selection” pinning site 710 may be configured to have athird pinning strength greater than the second pinning strength (for theterminal pinning sites 708) or the first pinning strength (for theintermediate pinning sites 706). An “arm selection” pinning site 710 maybe an initial pinning site 710 for an arm 704, with a greater pinningstrength (e.g., a greater notch depth) than the pinning sites of anotherarm 704, so that increasing the write current magnetically saturates onearm 704 before unpinning a domain wall 750 from the initial armselection pinning site 710 in another arm 704. A second arm 704 b, withan initial arm selection pinning site 710, may further include one ormore intermediate pinning sites 706 of the first pinning strength, and aterminal pinning site 708 of the second pinning strength, as in thefirst arm 704 a.

In another embodiment, a free layer 700 may include further arms 704extending from the nucleation or pad-shaped region 702. Further arms 704may include one or more intermediate pinning sites 706 of the firstpinning strength, and a terminal pinning site 708 of the second pinningstrength, as in the first arm 704 a, and may include initial armselection pinning sites 710 of increasing pinning strengths relative tothe third pinning strength (for the initial arm selection pinning sites710 in the second arm 704 b). Thus a controller may saturate a first arm704 a before unpinning a domain wall 750 b in the second arm 704 b, maysaturate the second arm 704 b before unpinning a domain wall 750 in athird arm 704, and so on.

In a certain embodiment, the intermediate pinning sites 706 may benotches with a notch depth that extends across approximately 20% of thewidth of an arm 704 (e.g., a depth of 15 nm to 25 nm for a 100 nm armwidth). In a further embodiment, the terminal pinning sites 708 may benotches with a notch depth that extends across approximately 40% of thewidth of an arm 704 (e.g., a depth of 35 nm to 45 nm for a 100 nm armwidth). In one embodiment, an initial arm selection pinning site 710 fora second arm 704 b may be a notch with a notch depth that extends acrossapproximately 60% of the width of an arm 704 (e.g., a depth of 55 nm to65 nm for a 100 nm arm width). A notch depth for an initial armselection pinning site 710 may be deeper than a maximum notch depth inanother arm 704, thus providing an initial arm selection pinning site710 with a greater pinning strength than pinning sites 706, 708 inanother arm 704. Various configurations of notch depths, or other shapesof pinning sites 706, 708, 710 configured to provide different pinningstrengths, will be clear in view of this disclosure.

FIGS. 7A-7H depict resistance states for an eight-state MTJ 500, inorder from highest resistance to lowest, as domain walls 750 are formedand moved by application of a write current. In FIG. 7A, the free layer700 is a single domain, magnetized antiparallel to the reference layer506, and the MTJ 500 is therefore in its highest resistance state. InFIG. 7B, the controller has applied a write current, forming aparallel-magnetized domain in the pad-shaped region 702, and expandingthe new domain to saturate the pad-shaped region 702, until a firstdomain wall 750 a is pinned at a pinning site 706 in the first arm 704a, and a second domain wall 750 b is pinned at an initial arm selectionpinning site 710 in the second arm 704 b. The formation and expansion ofthe parallel-magnetized domain lowers the resistance of the MTJ 500 toan intermediate state.

In FIGS. 7B, 7C, and 7D, the controller has applied increased writecurrents, causing the domain wall 750 a in the first arm 704 a to unpinfrom pinning sites 706, 708, and to move to successive pinning sites706, 708. Thus, the parallel-magnetized domain expands in steps,corresponding to further intermediate resistance states for the MTJ 500.In FIG. 7E, a further increase to the write current has unpinned thedomain wall 750 a from the terminal pinning site 708 in the first arm704 a, and expanded the parallel-magnetized domain to saturate the firstarm 704 a, resulting in a further decrease in resistance for the MTJ500.

In FIG. 7F, with the first arm 704 a saturated, a further increase tothe write current causes the domain wall 750 b in the second arm 704 b,which has been pinned at the initial arm selection pinning site 710since the parallel-magnetized domain first expanded into the second arm704 b, to unpin from the initial arm selection pinning site 710, and tomigrate to an intermediate pinning site 706. In FIGS. 7G and 7H, furtherincreases in the write current unpin the domain wall 750 b from pinningsites 706, 708, and expand the parallel-magnetized domain further alongthe second arm 704 b, to repin the domain wall 750 b at subsequentpinning sites 708 or to saturate the second arm 704 b. Thus, theparallel-magnetized domain is further expanded, resulting in furtherdecreases in resistance for the MTJ 500, until the MTJ 500 reaches itslowest resistance state in FIG. 7H.

In certain embodiments, a write current in the opposite direction, or anerase current, may be applied to form an antiparallel domain and may beincreased to saturate the arms 704 (through successive unpinning andrepinning), restoring the MTJ 500 to a highest-resistance orantiparallel state. In another embodiment, an MTJ 500 may be erased to aparallel state, and programmed to successively higher resistance statesby formation and expansion of an antiparallel domain. In a certainembodiment, an MTJ 500 may be rewritten without erasing, by applying areversed write current to move an existing domain wall 750 toward,rather than away from, the nucleation or pad-shaped region 702.

In various embodiments, a number of states for a multi-state magneticmemory cell or MTJ 500 is based on a number of notches or pinning sites706, 708, 710 for the one or more arms 704. In the depicted embodiment,the two arms 704 and the depicted notches or pinning sites 706, 708, 710provide eight states, allowing the MTJ 500 to store three bits of data.In a further embodiment, increasing the number of arms 704 or the numberof notches or pinning sites 706, 708, 710 for the free layer 700 wouldincrease the number of resistance states for the MTJ 500.

FIG. 8 depicts another embodiment of a free layer 800 for a magnetictunnel junction 500, in a top view. In the depicted embodiment, the freelayer 800 includes a nucleation region 802, arms 804, and pinning sites806, 808, 810, 816, which may be substantially as described above. FIG.8 illustrates a variety of embodiments of pinning sites 806, 808, 810,816, including v-shaped notches, u-shaped notches (in either edge of anarm 804), holes 816, or the like. In the depicted embodiment, initialarm selection pinning sites 810 are successively deeper notches (withincreasing pinning strengths) in successive arms. For example, theinitial arm selection pinning site 810 a for the second arm 804 b isdeeper than any of the notches in the first arm 804 a, the initial armselection pinning site 810 b for the third arm 804 c is deeper than theinitial arm selection pinning site 810 a for the second arm 804 b, andthe initial arm selection pinning site 810 c for the fourth arm 804 d isdeeper than the initial arm selection pinning site 810 b for the thirdarm 804 c. Thus increasing write currents will result in saturation ofthe first arm 804 a, then the second arm 804 b, then the third arm 804c, than the fourth arm 804 d. Various further embodiments of free layers800 may include various further numbers of arms 804 extending at variousangles from a nucleation region 802, various numbers pinning sites 806,808, 810, 816 of different sizes and shapes.

FIG. 9 is a schematic flow chart diagram illustrating one embodiment ofa method 900 for writing data to magnetoresistive random access memory.The method 900 begins, and a controller 206 applies 902 a first writecurrent to an MTJ 500 to form a domain wall in a nucleation region ofthe free layer 502. The domain wall may migrate to a first pinning sitein the free layer 502. The controller 206 applies 904 a second writecurrent to move the domain wall from the first pinning site to a secondpinning site in the free layer 502, and the method 900 ends.

FIG. 10 is a schematic flow chart diagram illustrating one embodiment ofa method 1000 for writing data to magnetoresistive random access memory.The method 1000 begins, and a controller 206 applies 1002 an erasecurrent to an MTJ 500 to magnetically saturate the free layer 502 in anerased state with no domain wall. The erased state may be a parallel orantiparallel state. The controller 206 applies 1004 a write current toform a domain wall in the free layer 502. The controller 206 determines1006 if a desired resistance state for the MTJ 500 (corresponding to adata value to be programmed to the MTJ 500) has been reached. If the MTJ500 is in the desired resistance state, the method 1000 ends.

If the MTJ 500 is not in the desired resistance state, the controller206 increases 1008 the write current, and applies 1010 the increasedwrite current to move the domain wall in the free layer 502. Thecontroller again determines 1006 if the desired resistance state for theMTJ 500 has been reached, and accordingly either continues the method1000 by increasing 1008 the write current, or ends the method 1000.

A means for forming a magnetic domain wall in a free layer for amagnetic tunnel junction, in various embodiments, may include anucleation region 602 of a free layer 600, a pad-shaped region 702 of afree layer 700, a center dot of a free layer, or the like. Otherembodiments may include similar or equivalent means for forming amagnetic domain wall.

A means for pinning a domain wall at one or more predetermined locationsalong one or more arms in a free layer, in various embodiments, mayinclude one or more pinning sites, one or more notches, one or moregaps, holes, or formed irregularities in the arms, or the like. Otherembodiments may include similar or equivalent means for pinning a domainwall

A means for modifying a resistance state of a magnetic tunnel junction,in various embodiments, may include a controller 206, row circuits 202,column circuits 204, row/axon circuits 402, column/dendrite circuits404, or the like, other logic hardware, and/or executable code stored ona computer readable medium. Other embodiments may include similar orequivalent means for modifying a resistance state of a magnetic tunneljunction.

The present disclosure may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the disclosure is, therefore,indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

What is claimed is:
 1. An apparatus comprising: a magnetic tunneljunction (MTJ) for storing data, the magnetic tunnel junction comprisinga reference layer, a barrier layer, and a free layer, the barrier layerdisposed between the reference layer and the free layer, the free layercomprising: a nucleation region configured to form a magnetic domainwall; and an arm, narrower than the nucleation region and extending fromthe nucleation region, the arm comprising a plurality of pinning sitesformed at predetermined locations along the arm for pinning the domainwall.
 2. The apparatus of claim 1, wherein the plurality of pinningsites of the arm comprises one or more intermediate pinning sites of afirst pinning strength and a terminal pinning site of a second pinningstrength, the second pinning strength greater than the first pinningstrength.
 3. The apparatus of claim 2, wherein the free layer furthercomprises a second arm extending from the nucleation region, the secondarm comprising an initial arm selection pinning site of a third pinningstrength, the third pinning strength greater than the second pinningstrength.
 4. The apparatus of claim 3, wherein the second arm furthercomprises one or more intermediate pinning sites of the first pinningstrength and a terminal pinning site of the second pinning strength. 5.The apparatus of claim 4, wherein the free layer further comprises oneor more further arms extending from the nucleation region.
 6. Theapparatus of claim 5, wherein the one or more further arms comprise oneor more intermediate pinning sites of the first pinning strength, one ormore terminal pinning sites of the second pinning strength, and one ormore initial arm selection pinning sites of increasing pinning strengthsrelative to the third pinning strength.
 7. The apparatus of claim 1,wherein the domain wall forms at the nucleation region in response to awrite current.
 8. The apparatus of claim 7, wherein the domain wallmigrates along the arm from one pinning site to a subsequent pinningsite in response to an increased write current.
 9. The apparatus ofclaim 7, wherein the write current controls a magnetic field applied tothe free layer.
 10. The apparatus of claim 7, wherein the write currentcontrols a spin current injected into the free layer.
 11. The apparatusof claim 7, further comprising a controller configured to supply thewrite current.
 12. The apparatus of claim 1, wherein a resistance statefor the MTJ is based on where the domain wall is pinned.
 13. Theapparatus of claim 12, wherein the MTJ provides four or more possibleresistance states.
 14. The apparatus of claim 1, wherein the MTJ isconfigured as a multi-weight synapse that couples two artificial neuronsin a neuromorphic computing array.
 15. A system comprising: aneuromorphic computing die, the neuromorphic computing die comprising aplurality of artificial neurons and a synapse array of multi-statemagnetic memory cells coupling the artificial neurons, wherein amagnetic storage layer for a multi-state magnetic memory cell comprises:a pad-shaped region for forming a magnetic domain wall; and one or morearms, narrower than the pad-shaped region and extending from thepad-shaped region, the one or more arms comprising a plurality ofnotches for pinning the domain wall.
 16. The system of claim 15, furthercomprising a controller that modifies synapse weights for the synapsearray by controlling write currents for the multi-state magnetic memorycells.
 17. The system of claim 15, wherein a number of states for amulti-state magnetic memory cell is based on a number of notches for theone or more arms.
 18. The system of claim 15, wherein at least one ofthe one or more arms comprises an initial arm selection notch deeperthan a maximum notch depth for another arm.
 19. An apparatus comprising:means for forming a magnetic domain wall in a free layer for a magnetictunnel junction; and means for pinning the domain wall at one or morepredetermined locations along one or more arms in the free layer. 20.The apparatus of claim 19, further comprising means for modifying aresistance state of the magnetic tunnel junction by depinning the domainwall from a first location, and causing the domain wall to migrate andpin to a second location.
 21. An apparatus comprising: amagnetoresistive random access memory (MUM) die, the MRAM die comprisinga plurality of memory cells, a memory cell comprising a fixed layer, abarrier layer, and a free layer, the barrier layer disposed between thefixed layer and the free layer, the free layer comprising: a nucleationregion configured to form a magnetic domain wall in response to a writecurrent; a wall extension region configured to provide a plurality ofresistance states for the memory cell corresponding to positions of thedomain wall within the wall extension region; and a plurality of pinningsites formed at predetermined locations in the wall extension region andconfigured to pin the domain wall such that the domain wall migratesfrom one pinning site to a subsequent pinning site in response to anincreased write current.
 22. The apparatus of claim 21, furthercomprising a controller configured to apply write currents to the memorycells.
 23. The apparatus of claim 21, wherein the free layer furthercomprises a second wall extension region configured to provideadditional resistance states for the memory cell corresponding topositions of a second domain wall within the second wall extensionregion.
 24. A method comprising: applying a first write current to amagnetic tunnel junction to form a magnetic domain wall in a nucleationregion of a free layer of the magnetic tunnel junction, wherein thedomain wall migrates to a first pinning site in the free layer; applyinga second write current to the magnetic tunnel junction to move thedomain wall from the first pinning site to a second pinning site in thefree layer, wherein the pinning sites are formed at predeterminedlocations in the free layer.
 25. The method of claim 24, furthercomprising applying an erase current to the magnetic tunnel junction tomagnetically saturate the free layer in an erased state with no domainwall.